Scraper or Doctor blades made from metallic material (e.g., carbon steel, stainless steel) are well-known in the art. During the bar soap making process, soap pellets/noodles are passed through roll mills and compressed into “thin flakes”. The blade is used to scrape the thin flakes that adhered to the roll mills to form “compacted flakes”, which are then further refined and extruded into bar soaps.
A surprising and unexpected challenge was discovered when using a metal blade to manufacture bar soaps containing Zinc Pyrithione (ZPT), a broad-spectrum anti-microbial active. ZPT-containing soap flakes are typically white or light colored. It was discovered that unwanted metals found in the metal blade can transfer into the soap and/or onto the soap surface and complex with pyrithione to form a dark colored pyrithione precipitate that can cause significant “discoloration” (e.g., gray, green, blue or purple colors) (see FIG. 1A). This discovery was unexpected since the ZPT-containing soap flakes come in contact with the metal scraper for a very short duration (e.g., less than 1/100th sec). As shown by FIG. 2, the resultant discoloration may adversely affect the overall aesthetics of the ZPT-containing bar soaps and may give consumers a negative impression of the shipment or storage conditions or that the ZPT-containing bar soaps may be of inferior quality.
Additionally, ZPT instability in bar soaps can pose a problem. Metal ions can also be introduced into the bar soap manufacturing process as impurities in raw materials, or metallic parts of other manufacturing equipments (e.g., roll mills, pipes, nozzles, etc.). It is believed that the oxidative reaction involving transition metal cations from the metal blade may further aggravate ZPT instability and accelerate oxidative loss over time. As a result, there can be a noteable reduction of the anti-microbial property of ZPT-containing bar soaps made using a metal blade.
A solution would be to substitute the metal blade with plastic material. However, many plastic material such as, for example, polycarbonate (e.g., carbon fiber blades), are generally hard and therefore tend to be brittle, and likely will not perform well as doctor blades. The concern is that brittle plastics can chip and crack and pieces of the material from the plastic blades may end up getting into the soap as foreign. Therefore, it would not be suitable to substitute any sort of plastic material for the metal blade.
Thus, there is a need for an improved manufacturing apparatus for manufacturing bar soaps, preferably ZPT-containing bar soaps. In particular, the need is for a manufacturing apparatus comprising a non-metallic blade for scraping soap adhered to a roll mill. The need also exists for a process for manufacturing a ZPT-containing bar soap that will not discolor through the use of the manufacturing apparatus. It is also desirable that the bar soap manufactured using this manufacturing apparatus will generally retain a significant portion of its anti-microbial properties.